1. Field of the Invention
The present invention relates to a transmitter/receiver apparatus including a transmitter for intermittently generating a radio signal and a receiver for receiving the radio signal.
2. Description of the Prior Art
With a decrease in size and cost of a transmitter/receiver apparatus, various application products have recently been developed. In addition to conventional pagers (pocket bells) which have been widely used, various new systems have become commercially available. For example, a missing article alarming apparatus (e.g., Japanese Patent Laid-Open (Kokai) No. 60-200395) and a missing child alarming apparatus have become commercially available. The missing article alarming apparatus is designed to prevent valuables, such as a bag, a brief case, and a purse, from being mislaid or lost. This apparatus is designed such that a transmitter is attached to a valuable article, and a compact portable receiver is used to receive a radio wave from the transmitter. When the receiver cannot receive a radio wave from the transmitter, an alarm is generated. The missing child apparatus is designed such that a transmitter is carried by a child, and his/her parent carries a receiver so as to receive a radio wave from the transmitter. When the receiver cannot receive a radio wave from the transmitter, an alarm is generated.
These missing article alarming apparatus and missing child alarming apparatus basically have the same arrangement. According to such a conventional arrangement, as disclosed in Japanese Patent Laid-Open (Kokai) No. 60-200395, when a carrier signal Sc transmitted from a transmitter cannot be received by a receiver, an alarm is generated. In this system, therefore, a radio wave must be transmitted without a pause. However, if the transmitter keeps transmitting a radio wave, the service life of a battery is undesirably shortened because the current consumption of the transmitter is large. In addition, if the receiver is continuously operated, since the receiver also consumes a large amount of current, the service life of a battery is undesirably shortened.
If carrier signals Sc are set to have various frequencies fc in order to prevent radio interference, limitations on mass production are imposed. Therefore, the carrier signals Sc preferably have a single frequency fc in a state wherein only a limited number of types of frequencies are used. In this case, if the carrier signal Sc is continuously transmitted from the transmitter of a missing article alarming apparatus of a given user, radio interference may be caused when the transmitter of a missing article alarming apparatus of another user approaches the receiver of the missing article alarming apparatus of the given user. As a result, even if the transmitter of the missing article alarming apparatus of the given user is moved far away from the user, no alarm may be generated.
In order to solve the above-described conventional problems, the applicant of the present invention has proposed a transmitter/receiver apparatus of an intermittent synchronous reception system using intermittent signals in Japanese Patent Laid-Open (Kokai) No. 63-267025. This transmitter/receiver apparatus has the following arrangement.
In this apparatus, a transmitter includes an intermittent signal generating circuit for generating an intermittent signal having a predetermined period, and a switching means for converting a high-frequency transmission signal into an intermittent transmission signal by using the intermittent signal. A receiver includes a synchronization detecting circuit having an intermittent operation pulse forming circuit for generating an intermittent operation pulse having the same period as that of the intermittent signal so as to determine the presence/absence of the intermittent signal by synchronously detecting the intermittent signal, and a switch means to be controlled by a synchronization detecting signal from the synchronization detecting circuit. The switch means switches the receiver to a continuous operation mode or an intermittent operation mode in accordance with the presence/absence of the synchronization detecting signal. That is, the apparatus comprises the transmitter for transmitting an intermittent radio signal, and the receiver which is designed to continuously detect an intermittent radio signal from the transmitter only during an operation start period. Upon detection of the intermittent signal, the receiver is switched to the intermittent operation mode so as to synchronously detect the intermittent signal. If the distance between the transmitter and the receiver is increased, and no intermittent signal from the transmitter reaches the receiver, the receiver cannot synchronously detect the intermittent radio signal. As a result, a buzzer as an alarming means is operated to inform the user of the receiver that an article or a child is missing.
In addition to the transmitter/receiver apparatus proposed by the applicant of the present inventor, attempts have been made to realize a compact apparatus and a compact, lightweight power source by minimizing the operation time and decreasing the current consumption in such a manner that one or both of a transmitter and a receiver is or are intermittently operated or to effectively use a frequency source by performing transmission/receiption of data between a plurality of transmitters/receivers using the same frequency in such a manner that when transmission/reception of data is to be performed between the plurality of transmitters/receivers, each pair of transmitter and receiver performs a transmitting/receiving operation at a predetermined time.
For example, a telemeter for intermittently transmitting/receiving measurement data obtained at a remote location and a paging (pocket bell) system for selectively paging several millions of receivers using a single transmission frequency have been put into practice. For example, a time-divisionally selective paging system as disclosed in Japanese Patent Publication No. 48-39843 and the technique of pocket bells described in National Technical Report, Vo. 1.26, No. 1, February 1980, "New "POCKET BELL" Paging Receiver" are available.
All of the conventional transmitter/receiver apparatuses of an intermittent synchronous reception system for performing an intermittent operation use an intermittent signal having a predetermined period. These apparatuses are satisfactorily effective in terms of a reduction in current consumption and apparatus size, and are considerably effective in terms of prevention of radio interference. With respect to the prevention of radio interference, the conventional apparatuses are not effective enough, and hence radio interference often occurs.
The occurrence of radio interference will be described below with reference to FIGS. 1(a) to 1(d). FIGS. 1(a) to 1(d) respectively show the waveforms of intermittent signals transmitted from a transmitter of a synchronous reception system. FIG. 1(a) shows the waveform of an intermittent signal transmitted from a transmitter of a given user and having a predetermined period t. FIGS. 1(b), 1(c), and 1(d) respectively show the waveforms of intermittent signals from transmitter/receiver apparatuses used by other users near the given user. More specifically, FIG. 1(b) shows the waveform of an intermittent signal from a transmitter having the same period t as that of the transmitter of the given user. FIG. 1(c) shows the waveform of an intermittent signal from a transmitter having a period 2t twice that of the period of the transmitter of the given user. FIG. 1(d) shows the waveform of an intermittent signal from a transmitter having a period (1/2)t half of that of the period of the transmitter of the given user.
As shown in FIGS. 1(a) to 1(d), all of the conventional transmitter/receiver apparatuses of an intermittent synchronous reception system use intermittent signals having predetermined periods. Therefore, if intermittent signals having the same period t are used by the two apparatuses as shown in FIG. 1(b), it is natural that radio interference is caused between intermittent signals Sa1 and Sb1, Sa2 and Sb2, . . . when the phases of the signals come close to each other. Even in the case of the intermittent signal having the period 2t as shown in FIG. 1(c), radio interference may be caused between intermittent signals Sa1 and Sc1, Sa3 and Sc2, . . . . Even in the case of the intermittent signal having the period (1/2)t as shown in FIG. 1(d), radio interference may be caused between intermittent signals Sa1 and Sd1, Sa2 and Sd3, Sa3 and Sd5, . . . .
That is, in the intermittent synchronous reception system, radio interference can be considerably reduced as compared with the continuous reception system. However, since intermittent signals having predetermined periods are used, the phases of the signals tend to come close to each other. In addition, a given intermittent signal is liable to radio interference with an intermittent signal having a period of an integer multiple or an integer part of the period of the given intermittent signal. Therefore, problems are posed in terms of practical applications.
If such transmitter/receiver apparatuses are increasingly used in future, a large number of missing article alarming apparatuses may be simultaneously used in a crowded train, or a large number of missing child alarming apparatus may be used in a crowded place such as an amusement park or a zoo. In such a case, even if the above-described improvement is achieved, radio interference is still liable to occur. Therefore, a great demand has arisen for countermeasures against radio interference along with the widespread use of the apparatuses in future.